The S38 was used on Euro cat-equipped cars - same engine as the US spec. The M88/3 has higher compression and a few other differences (like the timing chain as Marek says), and about 30 more HP at the crank when new.

How much are they worth? It depends on overall condition and documentation, just like the car. They are expensive to rebuild, so unknown mileage/undocumented ones go for fairly cheap - maybe a couple of grand. If you have records that include recent valve adjustments, timing chain component replacement (especially for an M88/3) and crank hub replacement, they can be worth $7k or more. A full rebuild can cost $10k, and they aren't a DIY lump. They should be rebuilt by someone with experience (in Canada that'd be Markus Glarner).